ValidationSettings are not supported for 's3' endpoint

While using S3 as the DMS target , validation is not supported. + https://docs.aws.amazon.com/dms/latest/userguide/CHAP_Validating.html

You can only use validation, when you use AWS DMS with the below endpoints as Source or Target:

Oracle
PostgreSQL-compatible database (PostgreSQL or Amazon Aurora PostgreSQL)
MySQL-compatible database (MySQL, MariaDB, or Amazon Aurora MySQL)
Microsoft SQL Server
IBM Db2 LUW

Unfortunately, there is no feature to validate data with S3 as target at this moment. And that's because of the validation works with DMS. Whenever you try to compare records between any source and target using DMS, DMS runs a SELECT query on both Source and Target, and then compares the values retrieved from both ends. Since its not possible on S3 to run a SELECT query for comparison as its not a database, validation is not supported with S3 as target.

You should think of working on a custom solution to compare the records between PostgreSQL and S3.

AWS Database Migration Service (AWS DMS) now supports validation to ensure that data is migrated accurately to S3.

If data validation is enabled for the migration then validation begins immediately after a full load is performed for a table. In the CDC phase, data validation compares the incremental changes as they occur. During data validation, AWS DMS compares each row in the source with its corresponding row at the target, verifies the rows contain the same data, and reports any mismatches.

Amazon S3 target data validation - https://docs.aws.amazon.com/dms/latest/userguide/CHAP_Validating_S3.html